1. Field of the Invention
This invention relates to a digital signal magnetic recording/reproducing apparatus such as a digital video taperecorder (DVTR).
2. Description of the Prior Art
Various proposals which have been made in the prior art for providing a digital signal magnetic recording/reproducing apparatus utilize various forms of modulations for converting an input digital signal to a recording signal.
We have already proposed a new digital signal magnetic recording/reproducing apparatus utilizing multi-level quadrature amplitude modulation, having high utilization efficiency of frequency band and high recording rate (cf. T. Matsuta et al "A Study of High Efficiency Channel Coding for Digital VTR". IEICE (the Institute of Electronics, Information and Communication Engineers of Japan) 1988 spring national convention record, part C-2, 2-59, Mar. 28-31, 1988, Tokyo). Further, we have also proposed a new digital signal magnetic recording/reproducing apparatus utilizing multi-level quadrature amplitude modulation, having a relationship established between the frequency of an input clock signal that is synchronized with successive data values of the input digital signal and the frequency of the carrier used for modulation. This relationship has been achieved by making the carrier frequency equal to the input clock signal frequency multiplied by a factor (M/N), where N and M are positive integers (cf. M. Shimotashiro et al U.S. patent application Ser. No. 329,720 filed on Mar. 28, 1989).
In this apparatus, however, no consideration is given on the shift of the relationship between the carrier phase and the modulated signal phase caused by the magnetic recording and reproducing process, i.e., the electric-to-magnetic conversion by the recording of the electric signal on a magnetic recording medium with a magnetic recording head and the magnetic-to-electric conversion by the reproducing of the recorded signal from the magnetic recording medium with a magnetic reproducing head. In other words, the phase of the modulated signal or clock is shifted relative to the phase of the carrier when subjected to the magnetic recording and reproducing process. So, error rate of the reproduced signal is degraded.
On the other hand, in the conventional demodulating circuit of a multi-level quadrature amplitude modulated signal for a digital communication apparatus, carrier the wave and clock are reproduced separately (e.g., M. Kuwahara et al "digital microwave communication", Kikaku center, 133, May, 1984). In such a system, the demodulating circuit requires a long "lock-up" time to start reproducing the carrier wave and clock accurately.